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This restricted distribution makes them excellent indicators for investigating the impact of climate change on species that are unable to easily track changing climatic space (climatically suitable areas).

Why is this indicator important?

Scientific (bioclimatic) models (Pearson & Dawson, 2003) have been used to investigate the exposure of species to climate change, i.e. the degree to which suitable bioclimatic conditions may change for a certain region over a given time-period. Bioclimatic models compare a species’ known distribution to the present-day ‘baseline’ climate, to establish a predictive model, and then project the location and extent of suitable climate space using climate change scenarios. A species’ exposure can be determined as the degree to which suitable climate space is gained/lost, or the degree to which it shifts spatially, requiring rates of migration to track bioclimatic space.

Often bioclimatic models do not include information on species dispersal, and therefore the extent to which species are actually at risk under a changing climate remains equivocal. In many cases, species dispersal rates are inadequately known and they cannot be included as a reliable component in assessing risk. However, it is generally accepted that climate change poses a particular threat to species which occur in fragmented habitats (Travis, 2003); it is possible that such species may not be able to migrate between isolated habitat patches in order to track their suitable bioclimatic conditions.

One solution in addressing the effect of dispersal-limitation is to focus on species that are indicators of ancient woodlands. Numerous studies have demonstrated that these species share a particular set of traits making them both habitat specialists, and dispersal-limited (Hermy et al, 1999 and Kimberley et al, 2013). As a consequence, they tend to be restricted to ancient woodlands with long ecological continuity (i.e. stable habitat conditions over multiple generations of trees), and have limited colonisation into regenerated woodland, even after 100s years. Given the relatively rapid pace of human-induced climate change(Diffenbaugh & Field, 2013), it is a reasonable assumption that these dispersal limited species represent biodiversity which will struggle to track shifts in suitable climate, via migration between woodland patches in Britain’s landscape.

What is happening now?

Bioclimatic modelling has indicated that lichen epiphytes in the continental north-east of Scotland (referred to as the ‘eastern region’) may be particularly exposed to climate warming, leading to large projected losses of suitable climate space (Fig. 1 and Fig. 2). In contrast, there have been mixed results in the gain/loss of climate space for Scotland’s internationally important Atlantic rainforest epiphytic lichens which are found further to the west (referred to as the ‘western region’), with species appearing to gain or lose climate space in an unpredictable manner. The magnitude of the shift towards unfavourable bioclimatic conditions also tended to be greater within the eastern region and there was striking local variation with high to low risk sites being mixed in occurrence throughout this region (Fig. 2).

Figure 1.Risk scores for ancient woodland sites combining the change in environmental suitability, and woodland extent. 0 = low risk; 1 = high risk It can be seen that ancient woodland sites in Scotland’s eastern continental region had higher risk scores compared to those in Scotland’s western rainforest region.

Figure 2. Distribution of the risk posed by climate change for ancient woodland sites in the western (A.) and eastern (B.) regions

Interpretation of indicator trends

The vulnerability of ancient woodland lichen epiphytes to climate change is linked to the size (and connectivity) of ancient woodland sites. The number of different microhabitats within sites and the long term continuity of these microhabitats, upon which the survival of ancient woodland epiphytes depends, have been found to increase in larger sites. The size of woodland sites was similar in both eastern and western regions (see Figure 3), as such it is the magnitude of the shift towards unfavourable bioclimatic conditions in the eastern region which drives the higher risk to lichens in this area (see Figure 3).

The considerable local variation in risk levels between sites in the eastern region reflects individual site extent and local climate variability related to Scotland’s mountainous topography.